Turbine driven starters for starting gas turbine engines have been in use for a number of years. Examples of representative patents include U.S. Patent Nos. 2,909,166; 3,087,305; 3,238,721; 3,521,505, all of which are assigned to the assignee of the present invention, as well as U.S. Pat. No. 2,224,322. These patents depict a starter having a turbine adapted to receive motive gas flow producing output rotation of a shaft. The shaft is coupled by various types of gear means and possibly a clutch to an output shaft connected to the gas turbine engine. By this arrangement, the motive gas flow impinging on the turbine commences a start cycle wherein the rotating assembly within the gas turbine engine is rotationally driven up to a threshold speed required for initiating self-sustained combustion within the engine. Once self-sustained combustion exists within the gas turbine engine, the motive gas flow to the engine is shut off and the clutch within the starter decouples with the engine side of the clutch being driven by the engine.
Various clutches such as those described within the above cited patents have been utilized to engage/disengage the starter unit from the engine. Another patent which is particularly directed toward the clutch is U.S. Pat. No. 3,638,774, having a common inventor with the present invention, and herein incorporated by reference. The 3,638,774 patent describes a clutch having a set of pivotally mounted pawls carried on the output shaft which pawls are radially inwardly biased by leaf springs to engage ratchet teeth on a cylinder driven by the turbine. Upon attaining a desired disengagement speed, the centrifugal forces on the pawls may overcome the spring biasing force and the pawls will pivot out of engagement with the ratchet, thus disengaging the starter unit.
In certain circumstances a problem with the clutch of the 3,638,774 patent may occur during a restart of an engine which is coasting down from a rotational speed above the threshold disengagement speed. Under this circumstance, when the start cycle is initiated, the turbine and associated ratchet will rapidly accelerate to a rotational speed in excess of the threshold disengagement speed. While the engine is coasting down and the starter is accelerating, the forces exerted by the biasing springs and the actuator elements will generally not be large enough to overcome the centrifugal force preventing the pawls from engaging the ratchet. Thus, when the engine has slowed down tot he threshold disengagement speed and the pawls are biased into engagement with the ratchet, there will be a speed differential between the engine and the ratchet which will cause a deleterious jarring or "crash engagement" to occur, potentially damaging the clutch assembly.
It would therefore be beneficial to have a clutch means for a gas turbine engine starter unit designed such that the driving member will not attain a rotational speed in excess of the driven member and thereby eliminate the possibility of "crash engagements" of the starter unit.